Polarization-independent tunable liquid-crystal Fabry-Perot interferometer filters

Two methods are studied for reducing the polarization dependence of a tunable liquid-crystal Fabry-Perot interferometer filter (LC-FPI). One uses a twisted nematic (TN) liquid-crystal in a Fabry-Perot cavity. We have theoretically explained that the twist angle should be pi/2 + m pi (m = 0, 1, 2,...) to eliminate the polarization dependency. An approximate 5 nm polarization-independent tuning with a pi/2 TN LC-FPI cell was achieved. The other uses a polarization diversity configuration. We have fabricated a polarization-independent LC-FPI filter module with a Faraday rotator using polarization diversity double-pass configuration. This filter module has a 3 dB bandwidth of less than 0.5 nm, a 15 dB bandwidth of 1.7 nm, and a polarization-dependent loss (PDL) of 0.8 dB. The filter module was utilized in 10 Gbit/s transmission experiments to confirm its feasibility.